Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing the new design.

Within the drawing, the outermost broken line rectangle illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines, including the line graphs and all text and icons, illustrate portions of the graphical user interface that form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 